1. Field of the Invention
This invention relates generally to a bandgap reference voltage circuit and in particular to a bandgap reference voltage circuit having a modified Brokaw cell configuration with a folded cascode operational amplifier. This circuit, which can be advantageously implemented in CMOS technology, can provide optimal voltage regulation.
2. Description of the Related Art
In general, a reference voltage is provided to maintain a baseline voltage level for an electronic circuit. Of importance, other voltages, power levels, and/or signals within the electronic circuit rely upon this baseline voltage level. Therefore, this reference voltage must be as consistent and as precise as possible, even when exposed to varying conditions (e.g. temperature).
One type of reference voltage circuit is a bandgap reference voltage circuit. A bandgap reference voltage circuit is typically preferred over other reference voltage circuits because of its relative simplicity and elimination of zener diodes, which can generate undesirable noise. Of importance, a bandgap reference voltage circuit can generate a reference voltage commensurate with ever-decreasing system voltages. For example, a bandgap-reference voltage circuit can produce an output voltage that is approximately equal to the silicon bandgap voltage of 1.206 V with a zero temperature coefficient (TC).
FIG. 1 illustrates a basic bandgap reference voltage-circuit 100 that can generate differing current densities between matched transistors 102 and 104, thereby producing a ΔVBE across resistor 105. In one embodiment, resistors 101, 103, and 105 can have resistances of 600 Ohms, 6 k Ohms, and 600 Ohms, respectively. Bandgap reference voltage circuit 100 sums the VBE of transistor 106 with the amplified ΔVBE of transistors 102 and 104 to generate VR. The components have opposite polarity TCs, i.e. ΔVBE is proportional-to-absolute-temperature (PTAT), whereas VBE is complementary-to-absolute (CTAT). In this manner, the summed output VR, when it is equal to 1.205 V (i.e. the silicon bandgap voltage), the TC is effectively minimized.
Unfortunately, bandgap reference voltage circuit 100 suffers from load and current drive sensitivity. Moreover, reference voltage VR needs accurate scaling to provide useful voltage levels (e.g. 2.5 V, 5.0 V, etc.)
FIG. 2 illustrates a type of bandgap reference voltage circuit 200, which is commonly called a “Brokaw cell”. Brokaw cell 200 improves on bandgap reference voltage circuit 100 by including an operational amplifier 207, which provides additional drive capability as well as convenient voltage scaling.
In this embodiment, Brokaw cell 200 includes two emitter-scaled transistors 202 and 206 (which form the bandgap core) that operate at identical collector currents due to equal load resistors 201 and 205 and a closed loop associated with operational amplifier 207. Assuming a smaller VBE of transistor 202 (e.g. transistor 202 can have 8× area of transistor 206), resistor 203 in series with transistor 202 drops the VBE voltage. Resistor 204, in turn, drops a PTAT voltage V1 according to the following equation, wherein R204 and R203 refer to the resistances of resistors 204 and 203, respectively.V1=2×(R204/R203)×ΔVBE
Resistors 208 and 209 (e.g. laser-trimmed resistors) in combination with operational amplifier 207 can be used to scale voltage VOUT The bandgap reference voltage VZ is generated at the base of transistor 206 by summing VBE and V1.
FIG. 3 illustrates a shunt mode reference voltage circuit 310, which functions similarly to bandgap reference voltage circuit 100. In circuit 310, like transistors 314 and 321 can be operated at a current ratio of 5×, as determined by the ratio of resistances of resistor 320 to resistor 312. An operational amplifier can be formed by the differential pair (i.e. transistors) 317 and 318, current mirror 316, resistors 315 and 322, and drivers (i.e. transistors) 323 and 324. In closed loop equilibrium, this operational amplifier maintains the bottom ends of resistors 312 and 320 at the same potential. In the configuration of circuit 310, ΔVBE is generated across resistor 313, VBE is formed across transistor 314, and V1 is provided across transistors 311 and 312. The nominal bandgap reference voltage can be computed by summing VBE and V1.
Unfortunately, implementing bandgap reference voltage circuits 100 and 310 using bipolar technology may significantly decrease the amount of digital circuits that can be placed on the same integrated circuit (IC). Specifically, a bipolar transistor has a parasitic collector to the substrate, which would otherwise interfere with CMOS device operation. Therefore, bipolar and CMOS devices must be isolated, if provided on the same IC, to ensure functionality. In another embodiment, separate ICs can be provided for bipolar and CMOS devices, thereby also undesirably increasing wafer production cost.
In yet another embodiment, bandgap reference voltage circuits 100 and 310 can be manufactured using biCMOS technology. Unfortunately, using this technology can also effectively double the cost of producing a wafer. Specifically, biCMOS technology requires the use of several additional layers in the IC, thereby increasing production costs and also reducing yield.
Brokaw cell 200 (FIG. 2) can be implemented in CMOS technology. Unfortunately, operational amplifier 207 derives its source voltage from the input voltage VIN. In this configuration, i.e. with its control terminal coupled to VIN, any variation in input voltage can also affect amplifier 207, thereby adversely affecting the stability of the bandgap reference voltage VZ. Specifically, even a few millivolts of offset introduced in operational amplifier 207 can result in an inability to accurately detect the voltage differential between its positive and negative input terminals. This detection problem, called power supply rejection (PSR), can render Brokaw cell 200 inapplicable for any system in which the input voltage may vary. Unfortunately, most systems have some variation in input voltage, either intentionally or unintentionally.
Therefore, a need arises for a bandgap reference voltage circuit that can be manufactured in CMOS technology while preserving the accuracy of the bandgap reference voltage irrespective of input voltage variations.